1. Field of the Invention
The field of the present invention relates to devices and methods for generating light, and more particularly to electrodeless plasma lamps.
2. Background
Electrodeless plasma lamps provide point-like, bright, white light sources. Because they do not use electrodes, electrodeless plasma lamps often have longer useful lifetimes than other lamps. Electrodeless plasma lamps in the prior art have certain common features. For example in U.S. Pat. Nos. 4,954,755 to Lynch et al., 4,975,625 to Lynch et al., 4,978,891 to Ury et al., 5,021,704 to Walter et al., 5,448,135 to Simpson, 5,594,303 to Simpson, 5,841,242 to Simpson et al., 5,910,710 to Simpson, and 6,031,333 to Simpson, each of which is incorporated herein by reference, the plasma lamps direct microwave energy into an air cavity, with the air cavity enclosing a bulb containing a mixture of substances that can ignite, form a plasma, and emit light.
The plasma lamps described in these patents are intended to provide brighter light sources with longer life and more stable spectrum than electrode lamps. However, for many applications, light sources that are brighter, smaller, less expensive, more reliable, and have long useful lifetimes are desired, but such light sources until now have been unavailable. Such applications include, for example, streetlights and emergency response vehicles. A need exists therefore, for a very bright, durable light source at low cost.
In the prior art, the air-filled cavity of the electrodeless plasma lamp is typically constructed in part by a metal mesh. Metal mesh is used because it contains the microwave energy within the cavity while at the same time permitting the maximum amount of visible light to escape. The microwave energy is typically generated by a magnetron or solid state electronics and is guided into the cavity through one or more waveguides. Once in the air-filled cavity, microwave energy of select frequencies resonates, where the actual frequencies that resonate depend upon the shape and size of the cavity. Although there is tolerance in the frequencies that may be used to power the lamps, in practice, the power sources are limited to microwave frequencies in the range of 1-10 GHz.
Because of the need to establish a resonance condition in the air-filled cavity, the cavity generally may not be smaller than one-half the wavelength of the microwave energy used to power the lamp. The air-filled cavity and thereby, the plasma lamp itself has a lower limit on its size. However, for many applications, such as for high-resolution monitors, bright lamps, and projection TVs, these sizes remain prohibitively large. A need exists therefore for a plasma lamp that is not constrained to the minimum cavity sizes illustrated by the prior art.
In the prior art, the bulbs are typically positioned at a point in the cavity where the electric field created by the microwave energy is at a maximum. The support structure for the bulb is preferably of a size and composition that does not interfere with the resonating microwaves, as any interference with the microwaves reduces the efficiency of the lamp. The bulbs, therefore, are typically made from quartz. Quartz bulbs, however, are prone to failure because the plasma temperature can be several thousand degrees centigrade, which can bring the quartz wall temperature to near 1000° C. Furthermore, quartz bulbs are unstable in terms of mechanical stability and optical and electrical properties over long periods. A need exists, therefore, for a light source that overcomes the above-described issues, but that is also stable in its spectral characteristics over long periods.
In prior art plasma lamps, the bulb typically contains a noble gas combined with a light emitter, a second element or compound which typically comprises sulfur, selenium, a compound containing sulfur or selenium, or any one of a number of metal halides. Exposing the contents of the bulb to microwave energy of high intensity causes the noble gas to become a plasma. The free electrons within the plasma excite the light emitter within the bulb. When the light emitter returns to a lower electron state, radiation is emitted. The spectrum of light emitted depends upon the characteristics of the light emitter within the bulb. Typically, the light emitter is chosen to cause emission of visible light.
Plasma lamps of the type described above frequently require high intensity microwaves to initially ignite the noble gas into plasma. However, over half of the energy used to generate and maintain the plasma is typically lost as heat, making heat dissipation a problem. Hot spots can form on the bulb causing spotting on the bulb and thereby reducing the efficiency of the lamp. Methods have been proposed to reduce the hot spots by rotating the lamp to better distribute the plasma within the lamp and by blowing constant streams of air at the lamp. These solutions, however, add structure to the lamp, thereby increasing its size and cost. Therefore, a need exists for a plasma lamp that requires less energy to ignite and maintain the plasma, and includes a minimum amount of additional structure for efficient dissipation of heat.